An organism must satisfy seven specific criteria to be considered living. These criteria identify different bodily functions, nutrients needs and physical responses which an organism must display if it is to be considered among the living. Even if one category is left wanting, the organism must be considered dead or non-living.
Nutrition and Respiration
Living organisms take in nutrients from the environment around them to provide energy and continued growth. This can be accomplished in a number of ways. Plants take in nutrients from the soil and water while animals take in nutrients by consuming plants and sometimes other animals. Living organisms must also go through respiration, meaning they release energy obtained through food according to the Cambridge University Press. This energy is then used to power other characteristics of living organisms.
Movement and Excretion
All living organisms move. This can be easily seen in organisms with legs like lions and tigers though more subtle in plant life which moves very slowly. Organisms must also excrete waste to be considered living. Excretion is a necessary process when converting food into energy as not all components of food can be used by the body. Excretion is also used to remove toxic components from the bodies of living organisms.
Growth and Reproduction
Living organisms must grow and develop according to the Cambridge University Press. Growth is defined as a permanent increase in cell number and size which is fueled by the intake of nutrients. Living organisms must also display the ability to reproduce. Offspring can be produced through asexual or sexual reproduction.
Organisms must show sensitivity to stimuli to be considered alive. Stimuli can include light, sound, heat, cold, chemical substances and gravity. Organisms must also show a response to a given stimulus to be considered alive. Responses can include plants growing towards light sources and animals showing an innate fear or attraction to fire.